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Self-assembly of a calix[5]arene bearing a 12-aminododecyl pendant group on the lower rim into supramolecular oligomers through intermolecular iterative inclusion events is readily triggered by contact with acid solutions and is reversed to the amino monomer precursor by treatment with a base. 1H NMR data are consistent with the formation of head-to-tail assemblies derived from endo-cavity inclusion of the alkylammonium moiety. Diffusion NMR and light-scattering studies provide evidence for the presence of oligomers in solution and show that different counterions and concentrations result in different oligomer sizes, whereas ESI-MS and SEM investigations, respectively, indicate that self-assembly also takes place in the gas phase and in the solid state. The growth of these supramolecular oligomers is concentration-dependent; however, as a consequence of the saline nature of the monomer, it also shows a distinct counterion-dependence owing to ion-pairing/solvation effects.

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A series of tri-O-substituted 1,3-bridged calix[5]arene crown-6 ethers bearing alkyl, arylalkyl, alkoxyalkyl, and alkoxycarbonylmethyl residues at the lower rim and either (t)()Bu or H substituents at the upper rim have been synthesized. (1)H NMR studies have shown that p-tert-butylcalix[5]crowns, irrespective of the size and nature of their lower rim pendant groups, adopt preorganized conelike conformations, whereas p-H-calix[5]crowns with bulky substituents preferentially exist in solution as partial cone conformers (C(1) symmetry). Calix[5]crown derivatives behave as mono- or ditopic receptors for isomeric butylammonium ions, forming endo-cavity (inside the calixarene cup) and/or exo-cavity (at the crown ether moiety) 1:1 complexes according to the shape of the guest. These two binding modes can be clearly distinguished and monitored by (1)H NMR titration experiments.

Dedicated to Professor Francesco Bottino on the occasion of his 70th birthday Cation ± p-electron interactions are known to play an important role in the recognition of positively charged guests by the electron-rich p-systems of natural [1] and synthetic hosts. [2] Calixarenes, [3] when suitably functionalized, can provide three-dimensional, rigid, p-rich cavities for the selective inclusion of organic cations in nonpolar media. [4] Recently we have shown that triether derivatives of (1,3)-ptert-butylcalix[5]arene crown-6 are able to discriminate between linear and branched alkylammonium ions by means of endo complexation. [5] The low association constants (K ass 48 ± 86 m À1 ) observed for the selective inclusion of nBuNH 3 ions are probably related to the shape of the cavity of these host molecules (C s symmetry) and their pronounced tendency to fill the cavity by canting a tert-butylphenyl residue inwards. [5] To develop more powerful receptors, these calixarenes with distorted cone conformations should be replaced by compounds with an improved shape and preorganized, permanent cone conformations. Although the shape of the calix[5]arene cavity can, in principle, be tuned by changing the nature and bulkiness of substituents on both the upper and lower rims, [6] the presence of tert-butyl substituents on the upper rim is essential to keep the molecule in a cone conformation and ensure selective inclusion of RNH 3 ions. Therefore, structural modifications to the lower rim of p-tert-butylcalix[5]arene (1 a) have been addressed. We have found that the attachment of long-chain pendant groups (bulkier than ethoxyethoxy) [7] on the lower rim of 1 aÐby exhaustive alkylation with appropriate electrophilesÐproduces derivatives 1 b ± e, which have a fixed and quite regular C 5v cone conformation. These receptors bind linear RNH 3 ions much more efficiently and selectively than the neutral synthetic host systems reported to date.Host ± guest binding affinities were investigated by 1 H NMR titration experiments of CDCl 3 /CD 3 OD (9/1, v/v) solutions of 1 b ± e with the four isomeric butylammonium picrate salts. Upon addition of increasing amounts of salts, the spectra consistently showed, from the first aliquot onwards, distinct sets of signals for the free and complexed species, indicating that exchange between the complexed and uncomplexed guests is slow on the NMR time scale. Consequently, the formation of 1:1 inclusion complexes was directly assessed by comparing the signal intensities of solutions containing equimolar amounts of host and guest. endo Complexation is unambiguously supported by the dramatic upfield shifts (complexation-induced shifts, CIS, up to Dd 4.1) of the resonances of the cavity-included alkyl chain of the guest. Due to the high preorganization of the host cavities prior to guest inclusion, the CIS of the receptors are lower (Dd 0.1 ± 0.4).The percentage of endo complexation for each butylammonium salt was determined by direct 1 H NMR analysis of equimolar solutions (5 Â 10 À3 m) of host and gu...